Recently in applications such as label package doubling as a protection of a glass bottle and a PET bottle etc. and display of articles, cap sealing and accumulation package, there have been widely used drawn films (so-called heat-shrinkable films) composed of a polyvinyl chloride resin, a polystyrene resin, a polyethylene resin or the like. Of these heat-shrinkable films, a polyvinyl chloride film has problems that heat resistance is low, and it generates hydrogen chloride gas in incineration and causes dioxin. A polystyrene film has problems that it is inferior in chemical resistance, as well as an ink with a special composition needs to be used in printing, it requires high temperature incineration and generates a lot of black smoke accompanied by an abnormal odor. Therefore, as a shrink label, there has been widely used a polyester-based heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in solvent resistance, and the use amount tends to increase being accompanied by an increase in turn volume of PET containers.
However, with regard to the conventional heat-shrinkable polyester film, further improvement in its shrinkage properties has been desired. In particular, as compared with the heat-shrinkable polystyrene film, at the time of covering a container such as a PET bottle, a polyethylene bottle or a glass bottle with a film and shrinking the film, irregularity of shrinkage and wrinkles may be generated, and characters and designs which are printed on the film may be distorted, and there has been a demand on the user side for reducing the distortion as much as possible.
Incidentally, at the time of using a heat-shrinkable film in the process of covering a container, designs and the like are printed on the film as necessary, after which the film is formed into a shape of a label, a bag or the like to be attached to the container, and the label or the like is shrunk by heat with a heating apparatus called a shrinkage tunnel to be closely adhered to the container. Examples of this shrinkage tunnel include a steam tunnel for blowing steam and shrinking a film by heat and a hot air tunnel for blowing hot air and shrinking a film by heat.
The steam tunnel is more satisfactory in heat transfer efficiency than the hot air tunnel, is capable of shrinking a film more uniformly by heating, and is also capable of attaining a satisfactory finishing appearance. However, there has been a problem that even if a steam tunnel is used for a polyester film, the finish properties are slightly poor as compared with the case of a polyvinylchloride film or a polystyrene film.
Furthermore, there has also been a problem that when a hot air tunnel easily causing temperature unevenness rather than the steam tunnel is used and a polyester film is shrunk, shrinkage whitening, irregularity of shrinkage, wrinkles, distortions and the like are easily generated, and the film is poorer in finish properties than a polyvinylchloride film or a polystyrene film.
For the above-mentioned reasons, there has been proposed a method of incorporating a polyester elastomer in a polyester resin as the film raw material in order to improve the shrinkage finish properties of a heat-shrinkable polyester film (Patent Document 1).
However, when the heat-shrinkable polyester film described in Patent Document 1 is used as a label for a bottle such as a polyethylene-made bottle which easily expands by heat, there has been a problem that the slack of the label is generated since the bottle is cooled to room temperature or so and then the size of the bottle which has bee expanded in heating returns to its original size, even if the label has been closely adhered to the bottle at the time of being shrunk by heating, and this is not preferred from the viewpoints of the performance and the appearance. Moreover, since the heat-shrinkable polyester film described in Patent Document 1 is almost not drawn in the longitudinal direction which is a direction orthogonal to the width direction as the main shrinkage direction, there has also been a problem that the mechanical strength in the longitudinal direction is low and moreover the perforation openability is poor.
With regard to the perforation openability, after the application of Patent Document 1, an investigation has been subsequently conducted by the present applicant and so on, and there has been successfully provided a heat-shrinkable polyester film excellent in perforation openability shown in Patent Document 2.
However, other problems have been generated. That is, in general, a heat-shrinkable film is rarely used immediately after produced and is often used after being aged (long-term storage) for reasons such as the storage and the transporting process. However, the heat-shrinkable film causes natural shrinkage during aging even at a temperature a little higher than ordinary temperature since the heat-shrinkable film is a film which shrinks by heat, lowering in performance occurs, and the shrinkage finish properties deteriorate.
For example, Patent Document 3 discloses a technique for enhancing the shrinkage finish properties while paying attention to the endothermic peak amount (enthalpy relaxation amount) around the glass transition point of the film. However, the publication date of this technique is more than 10 years ago, and shrinkage finish properties at the level of the technique described in Patent Document 3 cannot meet the current user's demands.